[BUG] Updating title object in bokeh server does not work

[kasuteru]

I am not able to change the title text of a figure interactively. In the minimal working example below, the title stays "old title" even after clicking the button. In the non-interactive version, this works fine.

Bokeh version 1.1.0.
Running on Windows 10, Anaconda environment, bokeh serve --show opens Firefox browser.

from bokeh.models.annotations import Title
from bokeh.plotting import figure, show
from bokeh.models.widgets import Button
from bokeh.layouts import row
import numpy as np
from bokeh.io import curdoc
x = np.arange(0, 2*np.pi, np.pi/100)
y = np.sin(x)
p = figure(title="old title")
p.circle(x, y)

button = Button(label="Update available curves", button_type="success")
def on_button():
  t = Title()
  t.text = 'new title'
  p.title = t
  print("Clicked")
button.on_click(on_button)

curdoc().add_root(row(p, button))

The title text stays the same. No error / warning shown in the console. "Clicked" is printed, so the function is definitely executed.

[bryevdv]

You should change the text on an existing title:

def on_button():
  p.title.text = 'new title'

The best practice and guidance in general for Bokeh is always: make the smallest change possible. In this specific case that means updating the text on the existing title, not replacing the Title object wholesale (which is a much more cumbersome operation). This principle is how things are demonstrated across the board in bokeh docs and examples.

cc @mattpap That said we should figure out things in this case (or make Title objects themselves readonly so that they can only be updated not replaced... I am somewhat inclined to do this in several places for the future)

[kasuteru]

Thanks for the quick response. I initially tried to change the title itself, not realizing it is an object with a "text" attribute. I then got the idea to change the whole object from a stackoverflow thread, where this was the accepted (but not most upvoted) answer... Your solution works for me.

[bryevdv]

@kasuteru if you have a link to the SO answer I can edit it